The present invention relates to receivers. Specifically, the present invention relates to receivers which can locate the strongest signal carrying a selected type of program.
The Radio Data System (hereinafter referred to as RDS) broadcasting system is implemented in Europe. RDS broadcast transmissions incorporate an encoded list of frequencies, called an AF list, that carry the same programming. RDS receivers can automatically track frequencies carrying the same broadcast, using the AF list. Thus, when RDS receivers are used in mobile receivers, such as car radios, the same station can be received continuously without having to re-tune the receiver while driving.
Prior art receivers which have this capability come in two forms. One type uses two tuners. The first tuner receives an audio signal on one frequency, while the other tuner receives another frequency on the AF list corresponding to the frequency to which the first tuner is tuned. The signal-to-noise ratios (field strengths) of the two signals are measured, and the receiver automatically switches to the frequency with the greatest signal-to-noise ratio.
The other type of prior art receiver uses one tuner. It receives an audio signal and at certain intervals carries out a muting function. During the muting interval, a signal at a different frequency from the AF list is received. The signal-to-noise ratio of the new frequency signal is measured and compared to a present frequency signal. As above, the receiver will switch to the frequency of the signal with the highest signal-to-noise ratio.
In the one-tuner receivers, the timing of the muting intervals may be based on preset intervals. Alternatively, the muting interval can be performed when no audio signal is detected (silent time) or when the received signal-to-noise ratio of the signal received drops below a predetermined level.
Each of the prior art methods has distinct disadvantages. The preset interval method subjects the listener to periodic muting of the audio signal and noise generated by the switching system. This has an unpleasant and unnatural affect on the user. The silent-time method does not have the problem of muting, but silent times can be very rare during broadcasts containing background music, thus delaying the automatic tracking function. In the last method, a stronger station may not be selected because no tracking occurs only when the field strength drops below the predetermined level. Although the two-tuner receivers, noted above, avoid the drawbacks of the one-tuner units, they are more complex and have higher production costs.